Enclosed fume hoods are ordinarily designed with an access window having a vertically sliding sash. The exhaust of air from the hood is arranged so that the velocity of air entering through the window when open is maintained within specified limits. Since these hoods are ordinarily exhausted to a draft system designed to draw an essentially constant volume of air, it is necessary that the hood be designed to deliver essentially the same volume of air to the draft system whether the window is open or closed.
Two bypasses are provided to maintain this required flow of air when the window is shut. The lower bypass is always open and insures the required air velocity past the materials under treatment in the hood. The upper bypass is essentially closed by the vertically sliding sash when it is in its upper, or open, position so that almost all the air entering the hood enters through the window. The upper bypass is open when the vertically sliding sash is in its lower or closed position, thus allowing sufficient air to flow to the exhaust system to compensate for the reduced flow through the window, so that the balance of the exhaust system is maintained.
Thus the volume of air required to be exhausted by the system is determined by the requisite air velocity through the open window, even though the window may be open for accessing only a small part of the operating time. A considerable amount of heated or cooled room air is thus uselessly discharged when the window is closed, representing a substantial energy waste.